The human Mr 90,000 heat shock protein and the Escherichia coli Lon protein share an antigenic determinant.

1. A monoclonal antibody (TG7A) reacts with a Mr 90,000 mammalian protein, accumulating during virus infection and heat shock. 2. This protein is encoded by a member of the Mr 90,000 heat shock gene family present in a range of organisms form yeast to man. 3. The antibody also recognises a Mr 94,000 protein in E. coli which similarly accumulates in virus infection and heat shock. 4. This protein has been identified as the Lon protease of E. coli. 5. The shared epitope and similar stress inducibility of the two proteins suggests that a functional and/or evolutionary relationship exists between them.